Conventionally, a three-way catalyst capable of simultaneously performing oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of nitrogen oxide (NOx) has been used as the catalyst for purifying automotive exhaust gases. For example, catalysts obtained by supporting a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Pd) on a porous oxide support such as alumina (Al2O3) are widely known. In order to simultaneously and effectively purify CO, HC and NOx by the activity of the three-way catalyst, it is important that the ratio between air and fuel (air-fuel ratio A/F) supplied to the automotive engine is controlled to the near theoretical air-fuel ratio (stoichiometric ratio).
However, the actual air-fuel ratio fluctuates to the rich (fuel rich atmosphere) side or the lean (fuel lean atmosphere) side centering on the stoichiometric ratio depending on the driving conditions or the like of an automobile and in turn, the exhaust gas atmosphere also fluctuates to the rich side or the lean side. For this reason, high purification performance cannot be necessarily ensured only by a three-way catalyst. In order to enhance the exhaust gas purifying ability of the three-way catalyst by absorbing the fluctuation of the oxygen concentration in the exhaust gas, a material having a so-called oxygen storage/release capacity of storing oxygen when the oxygen concentration in the exhaust gas is high and releasing oxygen when the oxygen concentration in the exhaust gas is low, is used in the exhaust gas purifying catalyst.
Regarding the oxygen storage/release material, for example, ceria (CeO2) or a material based on ceria (CeO2), such as ceria-zirconia (CeO2—ZrO2) composite oxide, is known and widely used in practice. However, in order to achieve stable purification of an exhaust gas, an oxygen storage/release material having a higher oxygen storage/release capacity has been demanded and studied.
Japanese Unexamined Patent Publication (Kokai) Nos. 2005-087892 (JPP'892) and 2006-075716 (JPP'716) describe an exhaust gas purifying catalyst comprising an oxygen storage/release material consisting of a rare earth oxysulfate or oxysulfide represented by A2O2SO4 or A2O2S (A is a rare earth element) and a noble metal supported thereon. More specifically, JPP'892 discloses Pt/La2O2SO4 or Pt/La2O2S in which the noble metal is platinum (Pt) and the rare earth element is lanthanum (La), and JPP'716 discloses the exhaust gas purifying catalyst in which the noble metal is palladium (Pd) and the rare earth element is lanthanum (La), praseodymium (Pr), neodymium (Nd) or samarium (Sm). In these patent publications, the exhaust gas purifying catalyst comprising an oxygen storage/release material consisting of a rare earth oxysulfate or oxysulfide and a noble metal supported thereon is found to have an oxygen storage/release capacity about 8 times higher than that of conventional exhaust gas purifying catalysts which comprise an oxygen storage/release material consisting of a CeO2—ZrO2 composite oxide and a noble metal supported thereon.